We have recently observed homologous interference during the infection of chicken embryo cells with Sindbis virus. Cells infected with as few as one plaque forming unit become refractory to superinfection by Sindbis virus within minutes after infection. The establishment of interference requires the action of a specific viral gene: ultraviolet inactivated virus is incapable of establishing interference and one temperature- sensitive RNA-minus mutant of Sindbis virus does not interfere at the nonpermissive temperature. Preliminary results suggest that the interfering virus prevents the irreversible attachment of superinfecting virions. These observations have led us to postulate that an early viral protein specified by the first virus to be uncoated acts to alter the membrane preventing the further processing of superinfecting virus particles. The objectives proposed in this application include: 1. Defining how the virus induces the mechanism for exclusion. 2. Identifying differences between the membranes of uninfected and infected cells immediately after interference is established. 3. Characterization of the excluded superinfecting virus. 4. Determining a possible relationship between the establishment of interference and the generation of defective virus particles. 5. Defining whether interference is influenced by cell type, whether it operates against other RNA viruses and whether animals infected with nonreplicating virus (ts-mutants) protect mice by excluding virulent wild-type virus.